Titanium alloys having improved castability

ABSTRACT

In order to improve castability of a titanium alloy, 0.01-5 wt %, preferably 0.1-3 wt %, of bismuth is introduced into the titanium alloy, based on the weight of bismuth and the titanium alloy. The titanium alloy is suitable for making a dental casting or a medical implant by casting.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is a continuation-in-part application ofU.S. patent application Ser. No. 10/179,310, filed Jun. 26, 2002, whichis a continuation-in-part application of U.S. patent application Ser.No. 09/548,266, filed Apr. 12, 2000, now U.S. Pat. No. 6,572,815B 1. Theabove-listed U.S. Pat. No. 6,572,815B1 and application Ser. No.10/179,310 are commonly assigned with the present invention and theentire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a titanium alloy, and moreparticularly to a titanium alloy casting. The present invention providesa method to improve the castability of a titanium alloy, so that it ismore suitable for use in making a dental casting and a medical implant.

BACKGROUND OF THE INVENTION

[0003] Due to its lightweight, high strength-to-weight ratio, lowelastic modulus, superior chemical corrosion resistance, and excellentmechanical properties at high temperature up to 550° C., titanium andits alloys have been widely used on aerospace, chemical, sports, andmarine industries. Their superior biocompatibility also makes them idealas the primary materials used in dental and osteological restorations orimplants, such as artificial bone pins, bone plates, shoulders, elbows,hips, knees and other joints, and dental orthopraxy lines.

[0004] A number of methods for fabricating titanium and its alloys witha desired shape have been developed. Among these, precision casting isthe most difficult. Precision casting has the advantage that the castproduced has a near net shape, which greatly decreases the titaniumfabrication cost. Also, precision casting is particularly suitable forproducing objects with a small volume, high size accuracy, andcomplicated shape, for example in dental and osteological fields.Moreover, titanium and its alloys could even be utilized in many othereveryday products, if the difficulty in precision casting could besolved.

[0005] There are many factors that affect the process of the precisioncasting and the properties of castings. According to the researchresults issued by Luk et al., in Dent. Mater., 8, 89-99, 1992, thefactors include alloy composition, alloy density, alloy surface tension,casting temperature, investment material type, mold temperature, castingmachine type, casting surface area/volume ratio, and pouring angle. Thecastability test is the most frequently used method for assessingvarious titanium precision casting processes. Castability is the abilityof a molten alloy to completely fill a mold space. Castability is acombination factor, and there is no international standard for assessingit today. Since castability is affected by many factors, researchershave designed various test methods in accordance with various castpatterns for assessing the castability. The cast patterns include spiralwax molds, fibrous nylon lines produced by injection molding (Howard etal., JDR, 59, 824-830, 1985), saucer-like molds, cylindrical molds,rectangle sheets, nylon mesh, and taper molds (Mueller et al., J. ofProsth. Den., 69, 367, Abstr. 2072, 1993). A wax mold of a simulatedcrown has also been designed (Bessing et al., Acta OdontologyScandinavian, 44, 165-172, 1986).

[0006] Titanium is inherently difficult to cast due to its high meltingpoint and high reactivity. Its low density is another problem incasting. Therefore, the improvement of casting process is the main issueof titanium precision casting. The casting machines used at presentutilize argon as the protective atmosphere to prevent high temperaturereactions. Induction or arc is used as the heat source in order toshorten melting time as well as lessen high temperature reactivity. Atpresent, in order to increase the pouring force and to avoid castingdefect caused by poor flowability of the molten metal, the titaniumcasting machines can be roughly divided into the centrifugal castingtype, the vacuum-pressure type, and the centrifugal-vacuum pressuremixed type (Yoshiaki, Conference Paper, 1-7, Australia, 1995).

[0007] U.S. Pat. No. 6,572,815B1 discloses a technique to improve thecastability of pure titanium by doping an alloying metal in an amount of0.01 to 3 wt %, preferably 0.5 to 3 wt %, and more preferably about 1 wt%. Among various alloying metals used in this application bismuth isfound the most promising element.

[0008] U.S. Pat. No. 2,797,996 discloses titanium base alloys of highstrength and ductility, and also of contamination resistance and highstrength at elevated temperatures, which contain as essentiallyconstituents titanium and tin, together with one or more additionalmetals selected from the groups comprising alpha promoters, betapromoters and compound formers. A large number of Ti—Sn base alloys wereprepared in this patent, including ternary titanium alloys containing1-5 wt % Bi. However, there is no teaching as to the improvement ofcastability or reducing surface tension of pure titanium or a titaniumalloy.

[0009] U.S. Pat. No. 4,810,465 discloses a free-cutting Ti alloy. Thebasic alloy composition of this free-cutting Ti alloy essentiallyconsists of at least one of S: 0.001-10%, Se: 0.001-10% and Te:0.001-10%; REM: 0.01-10%; and one or both of Ca: 0.001-10% and B:0.005-5%; and the balance substantially Ti. The Ti alloy includes one ormore of Ti—S (Se, Te) compounds, Ca—S (Se, Te) compounds, REM-S (Se, Te)compounds and their complex compounds as inclusions to improvemachinability. Some optional elements can be added to above basiccomposition. Also disclosed are methods of producing the abovefree-cutting Ti alloy and a specific Ti alloy which is a particularlysuitable material for connecting rods. Bismuth up to 10% was suggestedin this free-cutting Ti alloy. However, there is no teaching as to theimprovement of castability or reducing surface tension of pure titaniumor a titanium alloy.

[0010] U.S. Pat. No. 5,176,762 discloses an age hardenable beta titaniumalloy having exceptional high temperature strength properties incombination with an essential lack of combustibility. In its basic formthe alloy contains chromium, vanadium and titanium the nominalcomposition of the basic alloy being defined by three points on theternary titanium-vanadium-chromium phase diagram: Ti-22V-13Cr,Ti-22V-36Cr, and Ti-40V-13% Cr. The alloys of the invention arecomprised of the beta phase under all the temperature conditions, havestrengths much in excess of the prior art high strength alloys incombination with excellent creep properties, and are nonburning underconditions encountered in gas turbine engine compressor sections.Bismuth up to 1.5% was suggested in this age hardenable beta titaniumalloy. However, there is no teaching as to the improvement ofcastability or reducing surface tension of pure titanium or a titaniumalloy.

SUMMARY OF THE INVENTION

[0011] A primary object of the present invention is to provide a medicaldevice made of a titanium alloy having an improved castability.

[0012] Another object of the present invention is to provide a method ofimproving a castability of a titanium alloy.

[0013] A further object of the present invention is to provide a methodof using a titanium alloy in making a medical device.

[0014] The present invention discloses a method for improving acastability of a titanium alloy comprising at least one alloy elementselected from the group consisting of Mo, Nb, Ta, Zr and Hf, said methodcomprising introducing about 0.01-5 wt % Bi into said titanium alloy,preferably 0.1-3 wt % Bi, based on the weight of Bi and said titaniumalloy.

[0015] Preferably, said titanium alloy further comprises at least oneeutectoid beta stabilizing element selected from the group consisting ofFe, Cr, Mn, Co, Ni, Cu, Ag, Au, Pd, Si and Sn.

[0016] Preferably, the titanium alloy is substantially free from V.

[0017] Preferably, the titanium alloy is substantially free from Al.

[0018] Preferably, said titanium alloy consists essentially of Ti andMo; Ti and Nb; Ti and Zr; Ti, Mo and Fe; Ti, Mo and Cr; Ti, Mo and Nb;Ti, Mo and Ta; Ti, Nb and Fe; Ti, Ta and Fe; Ti, Nb and Zr; Ti, Al andNb; Ti, Mo, Zr and Fe; or Ti, Mo, Hf and Fe.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be described in detail with referenceto the illustrated embodiments and the accompany drawings, in which:

[0020]FIG. 1 is a schematic drawing showing the copper mold used for thecastability test in the present invention;

[0021]FIG. 2 shows the effect on castability by doping 1 wt %, 3 wt %and 5 wt % of bismuth to commercially pure titanium (c.p. Ti) and a Tialloy containing 7.5 wt % Mo and the balance Ti (Ti-7.5Mo) according tothe present invention;

[0022]FIG. 3 shows the effect on castability by doping 1 wt %, 3 wt %and 5 wt % of bismuth to a Ti alloy containing 6 wt % Al, 4 wt % V andthe balance Ti (Ti6Al4V) according to the present invention; and

[0023]FIG. 4 shows the effect on castability by doping 1 wt % of bismuthto various titanium alloys according to the present invention, whereinthe numerals before the elements in the Ti alloys represent the weightpercentage thereof.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention provides a medical device made of abiocompatible titanium alloy composition having an improved castabilitycomprising:

[0025] (a) about 0.01-5 wt % Bi, preferably 0.1-3 wt % Bi, based on theweight of the alloy composition;

[0026] (b) at least one alloy element selected from the group consistingof Mo, Nb, Ta, Zr and Hf; and

[0027] (c) the balance Ti.

[0028] The present invention also provides a method of using a titaniumalloy composition in making a medical device comprising casting theabove-mentioned biocompatible titanium alloy composition.

[0029] Preferably, said alloy composition further comprises at least oneeutectoid beta stabilizing element selected from the group consisting ofFe, Cr, Mn, Co, Ni, Cu, Ag, Au, Pd, Si and Sn.

[0030] Preferably, said titanium alloy composition is substantially freefrom V.

[0031] Preferably, the titanium alloy composition is substantially freefrom Al.

[0032] Preferably, the titanium alloy composition consists essentiallyof Ti and Mo; Ti and Nb; Ti and Zr; Ti, Mo and Fe; Ti, Mo and Cr; Ti, Moand Nb; Ti, Mo and Ta; Ti, Nb and Fe; Ti, Ta and Fe; Ti, Nb and Zr; Ti,Al and Nb; Ti, Mo, Zr and Fe; or Ti, Mo, Hf and Fe, in addition to Bi.

[0033] Preferably, the medical device is a dental casting.

[0034] Preferably, the medical device is a medical implant.

EXAMPLE 1 c.p. Ti and Ti—Mo alloys doped with 1 wt %, 3 wt % and 5 wt %of Bi

[0035] In this example 0, 1, 3 and 5 wt % of bismuth of 99.5% in puritywere melted into a grade II commercially pure titanium (c.p. Ti) andTi-7.5Mo alloy containing 7.5 wt % of Mo and the balance Ti by using acommercial arc-melting vacuum/pressure type casting system (Castmatic,Iwatani Corp., Japan). Appropriate amounts of c.p. Ti, molybdenum andbismuth were melted in a U-shaped copper hearth with a tungstenelectrode. The melting chamber was first evacuated and purged withargon. An argon pressure of 1.8 kgf/cm² was maintained during melting.After solidification/cooling in the same chamber in argon atmosphere,the thin oxidized layer of the ingot was removed by grinding and theground surface was ultrasonically cleaned in alcohol. The ingot wasre-melted three times to improve chemical homogeneity.

[0036] Prior to casting, the ingot was re-melted again in an open-basedcopper hearth under an argon pressure of 1.8 kgf/cm². The molten alloyinstantly dropped from the open-based copper hearth into a copper moldlocated in a second chamber at room temperature via a pouring gatebecause of the pressure difference between the two chambers. As shown inFIG. 1, the pouring gate 20 has an inlet of 20 mm diameter and an outletof 10 mm diameter, and a thickness of 18 mm between the inlet and theoutlet. The copper mold 10 has two parallel needle-shaped cavities of 1mm×53 mm (diameter×length).

[0037] Cast lengths (a measure of castability) of undoped and Bi-dopedc.p. Ti as well as Ti-7.5Mo alloy are compared in FIG. 2. As shown inthe figure, when 1 or 3 wt % Bi was doped in c.p. Ti, the cast lengthincreased by about 12%. When 5 wt % Bi was added, however, thecastability value declined. This “up and down” phenomenon was observedin a more dramatic way in Ti-7.5Mo system. When 1 wt % Bi was doped inTi-7.5Mo alloy, the cast length largely increased by 34%. Again, whenlarger amounts of bismuth were added, the castability values decreased.

[0038] According to the theory of Ragone et al. [RAGONE, D. V. ADAMS, C.M., and TAYLOR, H. F. (1956) Some Factors Affecting Fluidity of Metals.AFS Trans., 64, 640.], addition of an alloy element to a pure metalalways lowers the fluidity (increasing viscosity) of the metal due tothe formation of dendrites that causes resistance to fluid flow at theearly stage of solidification. This factor might satisfactorily explainwhy the castability value decreased when a relatively large amount (3 or5 wt %) of bismuth was added. However, the dendrite factor could notexplain the increase in castability when only 1 wt % Bi was added.

EXAMPLE 2 Ti6Al4V Alloy Doped with 1 wt %, 3 wt % and 5 wt % of Bi

[0039] The procedures in Example 1 were repeated except that acommercially available Ti-6Al-4V alloy (Titanium Industries, Parsippany,N.J., USA) was used to replace c.p. Ti and Mo metals. The results areshown in FIG. 3.

[0040] From the measurement of casting lengths (a measure ofcastability, FIG. 3), it is interesting to note that the castability ofTi-6Al-4V alloy could be largely enhanced by almost 30% by the additionof 1 wt % Bi in the alloy, compared to that of undoped one. When alarger amount (3 or 5 wt %) of bismuth was added, however, thecastability value of Ti-6Al-4V was not improved.

EXAMPLE 3 Castability of Some Commercial Ti Alloys with 1 wt % Bi Dopedand without Bi Doped

[0041] The procedures for preparing the doped and undoped Ti-7.5Moalloys in Example 1 were repeated to prepare Ti7.5Mo—Fe alloys with 1 wt% Bi doped and without Bi doped except that an additional metal Fe wasadded in an amount of 1, 3 and 5 wt %, separately.

[0042] The procedures for preparing the doped and undoped Ti-7.5Moalloys in Example 1 were repeated to prepare Ti15Mo alloy with 1 wt % Bidoped and without Bi doped except that the amount of Mo added was 15 wt%.

[0043] The procedures in Example 1 were repeated except thatcommercially available alloys TMZF (12 wt % of Mo, 6 wt % of Zr, 2 wt %of Fe, and the balance Ti) (Titanium Industries, Parsippany, N.J., USA),Ti13Nb13Zr (13 wt % of Nb, 13 wt % of Zr and the balance Ti) (TitaniumIndustries, Parsippany, N.J., USA), Ti5Al2.5Fe (5 wt % of Al, 2.5 wt %of Fe and the balance Ti) (Titanium Industries, Parsippany, N.J., USA),Ti6Al7Nb (6 wt % of Al, 7 wt % of Nb and the balance Ti) (TitaniumIndustries, Parsippany, N.J., USA), and Ti7Mo7Hf1Fe (7 wt % of Mo, 7 wt% of Hf, 1 wt % of Fe and the balance Ti) (Titanium Industries,Parsippany, N.J., USA) were used to replace c.p. Ti and Mo metals. Theresults are shown in FIG. 4 together with the 1 wt % Bi doped andundoped c.p. Ti, Ti7.5Mo, Ti6Al4V alloys prepared in Examples 1 and 2.

[0044] From the measurement of casting lengths (a measure ofcastability, FIG. 4), it can be seen that the castability of Ti alloysenhanced by the addition of 1 wt % Bi in the alloy ranges from about 17%(Ti5Al2.5Fe) to about 115% (Ti7.5Mo5Fe), compared to that of undopedone, while the castability improvement for c.p. Ti by the addition of 1wt % Bi is only about 12%.

[0045] More examples of titanium alloys were prepared and thecastability thereof was evaluated following the procedures recited inExample 1. The results are show in the following Table 1 together withthose of the alloys prepared in Examples 1 and 3. TABLE 1 Improvement incastability (cast length) of Ti alloys due to the presence of Bi Tialloy Cast length Improvement in composition (wt %) (mm) cast length (%)Ti—7.5Mo 11.5 — Ti—7.5Mo—1Bi 15.4 33.9 Ti—7.5Mo—3Bi 13.6 18.3Ti—7.5Mo—5Bi 12.0  4.3 Ti—7.5Mo—1Fe 7.3 — Ti—7.5Mo—1Fe—1Bi 13.1 79.5Ti—7.5Mo—2Fe 8.3 — Ti—7.5Mo—2Fe—0.1Bi 11.1 33.7 Ti—7.5Mo—2Fe—0.5Bi 12.753.0 Ti—7.5Mo—2Fe—1Bi 13.5 62.7 Ti—7.5Mo—3Fe 6.9 — Ti—7.5Mo—3Fe—1Bi 12.682.6 Ti—7.5Mo—5Fe 6.8 — Ti—7.5Mo—5Fe—1Bi 14.5 113.2  Ti—7.5Mo—2Cr 12.5 —Ti—7.5Mo—2Cr—1Bi 13.7  9.6 Ti—15Mo 12.7 — Ti—15Mo—1Bi 16.2 27.6Ti—15Mo—3Bi 14.8 16.5 Ti—15Mo—5Nb 12.9 — Ti—15Mo—5Nb—1Bi 15.4 19.4Ti—15Mo—5Ta 12.0 — Ti—15Mo—5Ta—1Bi 13.0  8.3 Ti—15Mo—2Fe 8.2 —Ti—15Mo—2Fe—1Bi 9.8 19.5 Ti—15Mo—2Cr 12.3 — Ti—15Mo—2Cr—1Bi 16.7 35.8Ti—20Mo 12.6 — Ti—20Mo—1Bi 15.7 24.6 Ti—10Nb 10.8 — Ti—10Nb—1Bi 18.571.3 Ti—25Nb 10.5 — Ti—25Nb—1Bi 14.7 40.0 Ti—25Nb—2Fe 7.0 —Ti—25Nb—2Fe—1Bi 9.2 31.4 Ti—25Ta—2Fe 7.2 — Ti—25Ta—2Fe—1Bi 8.4 16.7Ti—35Nb 8.0 — Ti—35Nb—1Bi 11.2 40.0 Ti—12Mo—6Zr—2Fe 9.2 —Ti—12Mo—6Zr—2Fe—1Bi 11.1 20.7 Ti—13Nb—13Zr 9.2 — Ti—13Nb—13Zr—1Bi 14.557.6 Ti—5Al—2.5Fe 10.8 — Ti—5Al—2.5Fe—1Bi 12.6 16.7 Ti—6Al—7Nb 14.1 —Ti—6Al—7Nb—1Bi 17.2 22.0 Ti—7Mo—7Hf—1Fe 8.0 — Ti—7Mo—7Hf—1Fe—1Bi 10.531.2 Ti—30Zr 13.2 — Ti—30Zr—1Bi 14.1  6.7

What is claimed is:
 1. A medical device made of a biocompatible titaniumalloy composition having an improved castability comprising: (a) about0.01-5 wt % Bi based on the weight of the alloy composition; (b) atleast one alloy element selected from the group consisting of Mo, Nb,Ta, Zr and Hf; and (c) the balance Ti.
 2. The medical device as setforth in claim 1, wherein said alloy composition comprises 0.1-3 wt %Bi.
 3. The medical device as set forth in claim 1, wherein said alloycomposition further comprises at least one eutectoid beta stabilizingelement selected from the group consisting of Fe, Cr, Mn, Co, Ni, Cu,Ag, Au, Pd, Si and Sn.
 4. The medical device as set forth in claim 1,wherein said titanium alloy composition is substantially free from V. 5.The medical device as set forth in claim 1, wherein the titanium alloycomposition is substantially free from Al.
 6. The medical device as setforth in claim 2, wherein the titanium alloy composition consistsessentially of Ti and Mo; Ti and Nb; Ti and Zr; Ti, Mo and Fe; Ti, Moand Cr; Ti, Mo and Nb; Ti, Mo and Ta; Ti, Nb and Fe; Ti, Ta and Fe; Ti,Nb and Zr; Ti, Al and Nb; Ti, Mo, Zr and Fe; or Ti, Mo, Hf and Fe, inaddition to Bi.
 7. The medical device as set forth in claim 1 which is adental casting.
 8. The medical device as set forth in claim 1 which is amedical implant.
 9. A method for improving a castability of a titaniumalloy comprising at least one alloy element selected from the groupconsisting of Mo, Nb, Ta, Zr and Hf, said method comprising introducingabout 0.01-5 wt % Bi into said titanium alloy, based on the weight of Biand said titanium alloy.
 10. The method as set forth in claim 9, wherein0.1-3 wt % Bi is introduced into said titanium alloy.
 11. The method asset forth in claim 9, wherein said titanium alloy further comprises atleast one eutectoid beta stabilizing element selected from the groupconsisting of Fe, Cr, Mn, Co, Ni, Cu, Ag, Au, Pd, Si and Sn.
 12. Themethod as set forth in claim 9, wherein the titanium alloy issubstantially free from V.
 13. The method as set forth in claim 9,wherein the titanium alloy is substantially free from Al.
 14. The methodas set forth in claim 10, wherein said titanium alloy consistsessentially of Ti and Mo; Ti and Nb; Ti and Zr; Ti, Mo and Fe; Ti, Moand Cr; Ti, Mo and Nb; Ti, Mo and Ta; Ti, Nb and Fe; Ti, Ta and Fe; Ti,Nb and Zr; Ti, Al and Nb; Ti, Mo, Zr and Fe; or Ti, Mo, Hf and Fe.
 15. Amethod of using a titanium alloy composition in making a medical devicecomprising casting a titanium alloy composition comprising (a) about0.01-5 wt % Bi based on the weight of the alloy composition; (b) atleast one alloy element selected from the group consisting of Mo, Nb,Ta, Zr and Hf; and (c) the balance Ti.
 16. The method as set forth inclaim 15, wherein said alloy composition comprises 0.1-3 wt % Bi. 17.The method as set forth in claim 15, wherein said alloy compositionfurther comprises at least one eutectoid beta stabilizing elementselected from the group consisting of Fe, Cr, Mn, Co, Ni, Cu, Ag, Au,Pd, Si and Sn.
 18. The method as set forth in claim 15, wherein saidtitanium alloy composition is substantially free from V.
 19. The methodas set forth in claim 15, wherein the titanium alloy composition issubstantially free from Al.
 20. The method as set forth in claim 16,wherein the titanium alloy composition consists essentially of Ti andMo; Ti and Nb; Ti and Zr; Ti, Mo and Fe; Ti, Mo and Cr; Ti, Mo and Nb;Ti, Mo and Ta; Ti, Nb and Fe; Ti, Ta and Fe; Ti, Nb and Zr; Ti, Al andNb; Ti, Mo, Zr and Fe; or Ti, Mo, Hf and Fe, in addition to Bi.
 21. Themethod as set forth in claim 15, wherein said medical device is a dentalcasting.
 22. The method as set forth in claim 15, wherein said medicaldevice is a medical implant.